The invention relates to improvement in reliability of optical waveguide fiber couplers. More particularly, the invention relates to an optical waveguide fiber coupler having sealed airlines and a method of sealing the airlines.
It is known in the art that light may pass back and forth between optical waveguide fibers when the light carrying portions of the waveguides are maintained close to one another. An optical waveguide coupler which exhibits excellent optical and physical properties is made from a starting assembly which includes two or more waveguides having an uncoated portion of their length surrounded by a glass tube. One such coupler is the Corning Inc., MultiClad.TM. coupler which derives its name from this surrounding glass tube which in some ways serves as an extra clad layer. The glass tube and the enclosed waveguides are heated to a selected softening temperature and the assembly is stretched, thereby reducing the thickness of the waveguide fiber clad glass layer and thus bringing the light carrying portions of the waveguides into coupling proximity. The glass tube serves to strengthen the coupler, to better confine the light passing through the coupler, as well as to affect optical properties of the coupler.
Important functional properties of the coupler are:
insertion loss, which compares the power in one output fiber to the power in one input fiber; PA1 the excess loss, which compares the total output power to the total input power; PA1 the dependence of insertion loss and excess loss on wavelength, where the objective often is achromatic coupler performance; PA1 the splitting ratio among the outputs; and, PA1 the stability of the coupler in typical environments. PA1 the number and arrangement of the fibers in the tube; PA1 the glass composition; PA1 the details of the heating means, including the uniformity and intensity of the applied heat; PA1 the stretching speed; and, PA1 the finished length of the coupling region.
Recent work in the coupler art has shown that the presence of so called "airlines", i.e., elongated open regions along the coupler length, bounded by the inner surface of the tube and the clad layer of the waveguides surrounded by the tube, improves coupler performance with respect to control of maximum excess loss over a band of wavelengths. The airlines also reduce the wavelength dependence of insertion loss, i.e., the airlines improve the achromaticity of the coupler. However, the performance of couplers so constructed was found to degrade in certain environments, particularly those in which the coupler was exposed to hot, humid conditions. The couplers having airlines were found to be more susceptible to degradation than those which were essentially completely fused, i.e., free of airlines.
Thus it is an objective of the invention to retain the benefits of having airlines in the coupler and eliminate the environmental sensitivity of the couplers.